This invention relates in general to television tuning systems and is particularly directed to a microcomputer-controlled television tuning system having a limited RAM and ROM for storing tuning information for a limited number of preferred channels.
Tuning systems having memory locations for the storage and recall of channel tuning information are now commonly available in television receivers. In this type of system, upon the selection of a particular channel number, binary signals stored in the memory are "read out", converted to tuning information for controlling the frequency of the television receiver's local oscillator in tuning to the carrier frequency of the desired channel, and used to display the selected channel number on a digital display. The generation of the tuning and display information is generally accomplished by means of a microcomputer in combination with a Random Access Memory (RAM) which store the channel tuning and display information. Typically, each memory bit location within the RAM corresponds to a designated channel number, or tuning frequency. The microcomputer also includes a Read Only Memory (ROM) in which is stored the microcomputer's operating program for synthesizing the channel number tuning and display signals.
The cost of these memory devices and associated tuning systems increases with the information storage capacity of the memory device. Thus, a memory having a greater data storage capacity is generally more expensive than one having a smaller storage capacity. The large number of available VHF and UHF channels in combination with the ever-increasing number of available CATV channels has required large memory devices in expensive and complex tuning systems. For example, one memory device may be dedicated to CATV channels, while another memory device may store VHF and UHF channel tuning information. In addition, those memory devices currently in use are not generally utilized in an efficient manner. For example, most memory-type tuning systems store tuning information for all available channels in spite of the fact that most viewers tune to only a limited number of preferred channels. Even in those systems having a preferred channel tuning capability tuning information for all channels may be stored in memory with "skip" and "stop" commands associated with respective non-preferred and preferred channels. In a typical lower cost preferred channel tuning system, the "skip" and "stop" features are replaced by an " all channel" tuning operation in which the television receiver stops at all channels during sequential channel scan tuning. No video display will be presented for those channels not having an associated signal, in which case noisy video and audio signals will be presented. This arrangement, in combination with the time consuming stops, presents an irritating inconvenience for the viewer.
A compromise intended to limit the size of the memory required in a television tuning system involves the storage therein of a limited number of preferred channels. The rationale behind this compromise is that even though there are a large number of available television channels, most viewers typically watch only a relatively small number of preferred channels. Thus, the storage of tuning data for only a limited number of channels to which the television receiver may be rapidly and automatically tuned reduces the size of the memory and the cost of the associated tuning system. However, in some systems having a channel skip/save capability two memories are utilized, one for storing tuning information and a second, separate memory in which is stored information for identifying those channels which are to be either skipped or saved. All of the aforementioned tuning system approaches make inefficient use of the information storage portion of the tuning system.
Prior art memory tuning systems also typically require relatively involved procedures in the storage and recall of tuning data from the memory. For example, entry of preferred channel tuning data in the memory requires conversion of the preferred channel number to a bit address location for storage in RAM. Thus, each channel number corresponds to a predetermined bit location in RAM. In addition, the recall of preferred channel data upon selection of a preferred channel number may require not only scanning of the entire RAM array, but also converting of the RAM address of the preferred channel to tuning data necessary for tuning to the desired channel. Thus, an intermediate step is involved in the storage and recall of tuning data in the memory. This intermediate step requires additional microcomputer programming available only in more complex and expensive microcomputer chips.
Finally, even those memory type tuning systems in which tuning data for only a limited number of preferred channels is stored suffer from operating limitations. For example, if in the CATV mode of operation and the channel selector is scanned through an air channel, i.e., VHF or UHF, the channel selector will typically stop scanning at the designated air channel even though in the CATV mode of operation. But since the corresponding CATV channel is not a preferred, selected channel, a video image will not appear on the television receiver's cathode ray tube. This irritating consequence not only defeats the purpose of rapid, automatic channel scan selection, but also underscores the inefficiency of memory utilization in prior art memory type tuning systems.
U.S. Pat. Nos. 4,138,647 and 4,158,816 to Wine disclose memory type tuning systems for storing tuning information for a limited number of preferred channels. The former patent includes a memory for the storage of tuning information corresponding to a limited number of preferred tuning positions and decoder means for locating an unused memory location containing undesired information into which new information corresponding to a desired tuning position may be entered and comparison means for locating a duplicate memory location containing information corresponding to the same tuning position as the new information. In this tuning system a counter is sequenced through the addresses for all the memory locations to search for a match between the contents of a display counter and the display memory portion of one of the memory locations in tuning to a preferred channel. The latter patent discloses a tuning system having a memory with a plurality of memory locations for storing tuning information associated with various channels a viewer may select. An input complementor is provided for generating, in response to viewer control, the complement of a binary word stored in memory so as to identify the associated channel as a nonpreferred channel. This causes memory locations containing binary signals representing illegal channel numbers to be skipped during the normal channel selection mode of operation of the tuning system. U.S. Pat. No. 4,187,469 to Tanaka discloses a television receiver having two modes of operation in which, in a first programmed channel mode, each entered channel number is automatically searched for among the limited locations of a memory with the system stopping only at programmed channels. In a second, skipped channel mode, only unprogrammed channel numbers are produced and displayed to the viewer in order to indicate to the viewer the unused memory locations in which program tuning information may be stored.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing stack memory tuning for a television receiver in which a limited number of preferred channel numbers are stored in sequence in a stack memory and are recalled therefrom in numerical order during rapid channel scanning selection. This approach results in as much as a 75% reduction in required memory capacity and program length over currently available memory type television receiver tuning systems.